SKÚTUSTAÐIR, IcelandOverlooking the lava spires of Lake Mývatn, inside an old white farmhouse, in a jar by the window, slumped in a cloudy green bath, Iceland’s last living lake ball is slipping away.

The fluffy green globes—supersize diatoms as large as a head of cabbage—are one of the planet’s most unusual plants. Vast colonies once teemed in this lake renowned for its dramatic aquascape and abundant wildlife. “Lake balls” is their polite name; in Icelandic, they’re called “Kúluskítur,” a colorful epithet uttered by fishermen when the balls got tangled in their nets.

This distinctive form of the freshwater alga Aegagropila linnaei is exceedingly rare. Mývatn, one of Europe’s most important waterbird habitats, used to host the largest lake ball colonies on Earth. This